The present invention is a method for fabricating fiber-optic preforms with complex refractive-index and/or dopant distributions to a high degree of accuracy and precision. In particular, the present invention focuses on the fabrication of performs for providing rare-earth-doped optical fibers such as those widely used in fiber amplifiers and lasers.
The simplest method of preform fabrication is the so-called “rod-in-tube” method such as is disclosed and described in U.S. Pat. Nos. 4,668,263 and 4,264,347. A rod of glass that will form the core of the fiber is inserted into a thick-walled tube that will become the cladding, and the two are fused together at high temperature. The relative dimensions of the core and cladding in the drawn fiber are identical to that of the original preform. The main advantage of the rod-in-tube technique is its simplicity and as such it was used almost exclusively during the earliest years of fiber development. However, while simple, this technique was also quite limited in its ability to implement optical fiber designs having any but the most rudimentary characteristics, Newer methods capable of producing ultra-low-loss fibers, such as are required for optical telecommunications, have essentially replaced the rod-in-tube technique.
In order to practice the rod-in-tube method, bulk glass is usually synthesized by mixing together the various ingredients in powder form and melting the mixture in a high-temperature furnace. All modern preform fabrication methods, however, are based instead on vapor-deposition techniques. The core and cladding materials are formed by reacting various gas-phase precursors at high temperature to form a glass “soot” that is subsequently sintered into a solid material. A principle advantage of the vapor-deposition process is its inherent capacity for providing a built-in purification step that immediately precedes the synthesis step. Starting reagents (liquids or solids) are heated and delivered to a reaction zone as a vapor phase. This distillation-like process leaves behind the vast majority of contaminating species typically present as trace constituents in the starting reagent materials, most notably transition metals.
Three types of vapor-deposition processes have been developed for fabrication of fiber-optic preforms. By far the most widely used method in the manufacture of rare-earth-doped fibers is the so-called “Modified Chemical Vapor Deposition” (MCVD) process. In this technique, volatile compounds, usually halides or chelated complexes, containing the desired dopant species 1, as a gas phase, are reacted with oxygen within an inside portion 2 of a thick-walled silica reaction tube 3, as shown in FIG. 1. As reactants 1 are delivered, silica reaction tube 3 is rotated while its outside surface is heated with an oxygen-hydrogen flame 4. The flame is translated back and forth along the axis of the tube. Combustion of gas-phase reactants 1 is confined to heated zone 2a, inside the tube, and deposition of the products of combustion (“soot” 5) occurs on the inner surface 2a of silica reaction tube 3. Following the combustion/deposition step, the temperature in the tube is increased to ˜1500° C., which sinters the deposited soot 5 into a solid layer of material. The deposition and sintering cycle is then repeated to build up additional layers of glass, after which the temperature of the tube is raised to >2000° C., at which point surface tension causes the tube to slowly collapse inward to form a solid rod serving as the finished preform.
In the simplest version of MCVD, silica tube 3 forms the “cladding” of the preform (i.e., the region surrounding the core), and vapor-deposited material 5 forms the “core”. One of the main advantages of MCVD, however, is that the chemical composition of the glass can be varied as a function of its radial position in the preform. That is, by adjusting the mixture of dopant species as each successive layer is deposited, the composition of the core and, if desired, of the portion of the cladding formed by the deposition process can be customized for specific applications. This procedure can thereby be used to achieve a structured or graded dopant profile in the preform and thus a corresponding structured or graded refractive-index profile in the subsequently fabricated optical fiber.
An important variant of the standard MCVD process is a technique called “solution doping”, which provides an alternative method for introducing a dopant-oxide species into the preform. In this method variation, a soluble salt of one or more dopant species is dissolved in a suitable solvent, such as alcohol. The partially sintered glass soot is soaked in the salt solution, and the solvent is subsequently removed by evaporation. The sintering process then proceeds as before, consolidating the dopant species and host material into a solid glass preform.
Related to MCVD are two other vapor deposition processes, referred to as “Outside Vapor Deposition” (OVD) and “Vapor Axial Deposition” (VAD). In both techniques, a chloride of the desired dopant species 1 is introduced and reacted with H2O generated in an oxygen/hydrogen flame. Flame 4 is directed against solid substrate 6 where soot 5 is deposited. The substrate in the OVD process is a rotating silica rod, as shown in FIG. 2. When enough material has been deposited, the partially sintered boule of glass is removed from the silica rod and fully sintered. The sintered mass is then collapsed, as before, at high temperature to form the solid glass preform. In the VAD technique, torch flame 4 is directed onto the end of a rotating silica pedestal 7 as shown in FIG. 3. As with MCVD, solution doping can be used with the OVD and VAD processes to incorporate additional dopant species into the pre-sintered glass preform before the final sintering step is carried out. The main differences between the OVD and VAD techniques are:                1) The radial profiles of the dopant species (including rare-earth constituents and other species such as B, Al, P, Ge, and F), and therefore the refractive index, can be controlled more easily in the OVD process.        2) The VAD process eliminates the sometimes difficult step of removing the pre-sintered soot boule from the silica rod.        3) The VAD process does not require the preform-collapse step.        
A characteristic common to all vapor-deposition techniques is poor process control. Delivering known and stable concentrations of dopant precursor species is particularly difficult. The rare-earth chlorides, for example, must be delivered as vapor through heated delivery lines to avoid recondensation. In addition, these species are very reactive, making it difficult to use mass-flow controllers or similar devices to regulate reactant flow rates and therefore rates of species addition. Furthermore, fluctuations in the temperature distribution of the reaction zone affect the composition of the preform by changing the relative rates of the various oxidation reactions and by changing the soot deposition efficiency. Similarly, with the solution doping technique, the distribution of dopant species incorporated into the host material is often non-uniform and unpredictable (the density and pore size of the partially sintered glass network can vary substantially). In practice, it is usually necessary to adjust the various process parameters by trial and error, fabricating several preforms until one of acceptable quality is obtained. Where tolerances on refractive index and/or dopant concentration are important, or where the shapes of the required dopant and/or refractive-index profiles are complex, the probability of producing a preform having an acceptable level of quality decreases dramatically. As a result, the range of fiber designs that can be fabricated is quite limited. This limitation persists despite large investments of time and resources in the development of optical fibers for a wide variety of commercially significant applications {see S. E. Miller and A. G. Chynoweth eds., Optical Fiber Telecommunications (Academic Press, San Diego, Calif., 1979); P. C. Becker, N. A. Olsson, and J. R. Simpson, Erbium-Doped Fiber Amplifiers (Academic Press, San Diego, Calif., 1999)}.
The present invention is directed toward solving these problems by providing a technique wherein a plurality of rods is bundled and fused into a glass preform, which is subsequently drawn into an optical fiber. Related art includes the development of multicore optical fibers (U.S. Pat. Nos. 6,041,154; 5,706,825; 4,613,205; and 4,011,007), in which several cores share a common cladding, e.g., for passive image-transfer applications. Although the present technique provides the flexibility to fabricate similar structures (and many others), such “multiple fibers” are not the emphasis of this invention, nor do they have the novel properties of the fibers discussed below.